Sensing element arrangement for a fingerprint sensor

ABSTRACT

For each pixel in an array of pixels in a fingerprint sensor, a fingerprint capacitor is defined by the fingerprint-bearing skin of a user&#39;s finger proximate to the top exposed surface of a sensing plate of the pixel. First and second plates are embedded in dielectric material beneath the sensing plate, and define therewith first and second capacitors of a sensing element. The capacitance of the fingerprint capacitor is coupled by the sensing element to an amplifier. During a sensing operation, the amplifier generates a pixel output signal that is a function of the variable capacitance of the fingerprint capacitor, which varies according to the presence of a fingerprint ridge or valley appearing directly above the sensing plate when the user&#39;s finger is in contact with the fingerprint sensor.

PRIORITY

This application is a divisional patent application of U.S. patentapplication Ser. No. 09/997,549, filed Nov. 27, 2001 now U.S. Pat. No.6,927,581, issued Aug. 9, 2005.

BACKGROUND OF THE INVENTION

The present invention relates generally to capacitive fingerprintsensors, and more particularly to improvements in the structure ofsensing elements that capacitively interact with the fingerprint-bearingskin of a user's finger.

A capacitive distance sensor is disclosed in commonly-assigned U.S. Pat.No. 6,114,862 by Tartagni et al., the disclosure of which is herebyincorporated by reference. The Tartagni et al. patent discloses thebasic structure and operation of a conventional solid-state fingerprintsensor that is formed on a single semiconductor chip. FIGS. 1 and 2herein correspond to FIGS. 1 and 4 of the Tartagni et al. patent, andare briefly described below to facilitate an understanding of theapplication of the present invention in the context of a two-dimensionalfingerprint sensor array.

The present invention provides an improvement in the capacitive sensingelement that is replicated in each of the sensor cells or “pixels” thatare included in a two-dimensional array of sensor cells. Severalembodiments of the improved sensing element are described.

SUMMARY OF THE INVENTION

An improved sensing element for a capacitive fingerprint sensor includescapacitor plates at multiple levels separated by dielectric material,the plates including a surface plate disposed on a top surface of acomposite dielectric body, and spaced first and second plates disposedbeneath the surface plate and separated therefrom by portions of thecomposite dielectric body. The surface plate and the adjacent topsurface portions of the composite dielectric body define a sensingsurface to which the fingerprint-bearing skin of a user's finger isapplied during a sensing operation. The surface plate and proximateskin, which may be either a fingerprint ridge or valley, together definea fingerprint capacitor. The surface plate and underlying first andsecond plates define respective first and second capacitors. These twocapacitors are interconnected with active circuit elements that generatea pixel output signal, which is a function of the variable capacitanceof the fingerprint capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art fingerprint sensor over whichthe present invention provides an improvement;

FIG. 2 is an enlarged schematic cross section of a small portion of afinger positioned above two adjacent sensor cells of the device of FIG.1;

FIG. 3 is an enlarged schematic cross section of a small portion of afinger positioned above a sensor cell, showing capacitor plates in crosssection and capacitor symbols superimposed thereon to facilitate anunderstanding of the functionality of the structural elements inaccordance with the invention;

FIG. 4 is an enlarged schematic cross section similar to FIG. 3 showingan alternative circuit arrangement of elements in accordance with theinvention;

FIG. 5 is a circuit diagram of the capacitors that make up a basicsensing element according to the invention;

FIG. 6 is a generalized depiction of the sensing element of FIG. 5showing its nodes that connect with other circuit elements;

FIG. 7 is a circuit diagram corresponding to FIG. 3 showing one of twoalternative circuits according to the invention;

FIG. 8 is a circuit diagram similar to FIG. 7 and corresponding to FIG.4 showing the other of two alternative circuits;

FIGS. 9–12 are schematic cross sections of alternative embodiments ofsensing elements according to the invention, showing a surface capacitorplate and capacitor plates embedded therebelow in dielectric material;

FIG. 10A is an alternate view of the structure of FIG. 10 showingelectrical circuit elements overlaid on the cross-sectional platestructure; and

FIG. 10B is a plan view of an interdigitated layout of the embeddedcapacitor plates of FIG. 10A.

DESCRIPTION OF THE PRIOR ART

FIG. 1 shows a layout for prior art sensor device 1 for sensing adistance between a sensing surface of the device and thefingerprint-bearing skin of a user's finger. The sensor device 1includes a number of cells 2 arranged to form an array 3, each cellconstituting an elementary sensor or pixel. The simplicity of theindividual cells 2 enables the sensor device 1 to be implemented inintegrated form on a single semiconductor chip.

The sensor device 1 also comprises a horizontal scanning stage 5 and avertical scanning stage 6 for enabling one of the cells 2 at a timeaccording to a predetermined scanning pattern. The stages 5 and 6 enablethe outputs of the cells to be read using shift registers, addressdecoders, or other suitable circuitry.

The sensor device 1 also comprises a supply and logic unit 7, whichsupplies the circuit elements of the device with power (including thecells 2), feeds the necessary reference voltages, and controls thetiming of device operations. FIG. 1 shows that the supply and logic unit7 includes a voltage source 12. A buffer 8 is interconnected with theoutputs of all the cells 2, and includes an output 10 for sequentiallygenerating signals corresponding to the outputs of the cells 2 accordingto the sequence in which they are enabled by scanning the stages 5, 6.

FIG. 2 shows the details of two adjacent cells 2A and 2B with a skinsurface portion 18 of a human finger positioned thereover. The elementsof each of these two cells bear the designators A or B but areessentially identical, as will now be described. Each cell 2 preferablycomprises a low-power inverting amplifier 13 having an input node 16 andan output node 17, which also defines the output of individual cell 2.Each cell 2 also preferably includes first and second coplanar capacitorplates 23 and 24, positioned facing the skin surface 18 of the fingerbeing printed. The plates 23, 24 are covered with a dielectric layer 25having a sensing surface 26 that covers the face of the integratedsensor device 1, including the entire array 3 of cells 2. A reset switch19 is connected between the input node 16 and output node 17 of theinverting amplifier 13. An input capacitor 20 is connected between aninput node 21 of the cell 2 and the input node 16 of the invertingamplifier 13. The input node 16 of the inverting amplifier 13 also has aparasitic capacitance depicted by capacitor 30. Likewise, the outputnode 17 has a parasitic capacitance depicted by capacitor 31.

The skin surface 18 includes a ridge 39 contacting the sensing surface26 adjacent to the first cell 2A and a valley 38 disposed just above thesensing surface 26 adjacent to the second cell 2B. As a result, thefirst and second cells 2A, 2B will each produce different capacitivecoupling responses in the sensor device 1. Accordingly, the first cell2A will sense a smaller distance d1, signifying the ridge 39, than thesecond cell 2B, which senses a larger distance d2, signifying the valley38. The distance d2 sensed by the second cell 2B will be the average ofa distance d2 a between the first capacitor plate 23B and the portion ofthe skin surface 18 directly above the first capacitor plate 23B and adistance d2 b between the second capacitor plate 24B and the portion ofthe skin surface 18 directly above the second capacitor plate 24B. Froma lumped-model point of view, this structure realizes a three-capacitorscheme that can sense the difference between a contacting member, aridge, and a non-contacting member, a valley.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to FIGS.3–12, like elements in the various figures being designated by the samereference characters.

Referring to FIG. 3, one embodiment of a sensor cell or pixel is shownand designated generally by reference numeral 40. Each pixel 40 istypically one of many identical such pixels arranged in atwo-dimensional array formed on a semiconductor chip that is theessential component of a solid-state fingerprint sensing device. Pixel40 has an input node 42 and output node 44. The input node 42 receives apulsed input signal ΔV from a reference voltage source V_(r) during aread operation. The ΔV input signal preferably is applied to all pixelsin a selected column of the pixel array. For example, FIG. 1 showsvertical scanning stage 6 that selects individual columns of the array3, one column at a time. In FIG. 3, the output node 44 is connected tooutput circuitry 48 that senses output signals from the pixel 40periodically.

The output of pixel 40 is generated by an inverting amplifier A such asthat described in U.S. Pat. No. 6,114,862. The inverting amplifier A hasan input node 50 corresponding to its negative (−) input, and an outputnode corresponding to the pixel's output node 44. An input capacitorC_(input) interconnects the pixel's input node 42 with the invertingamplifier's input node 50. The inverting amplifier's positive (+) inputis connected to a voltage source, such as ground. The ground symbols inthe figures may represent the actual ground terminal of the fingerprintsensing device or may represent a virtual ground bus internal to thechip. A parasitic capacitor C_(pi) connects the inverting amplifier'sinput node 50 to ground, and a parasitic capacitor C_(po) connects theoutput node 44 to ground. Just prior to a sensing operation, a resetswitch 56 is opened to prepare the inverting amplifier A to generate anaccurate output signal during the sensing operation. The reset switch 56may be a conventional transistor, such as an NMOS transistor, andinterconnects the inverting amplifier's input node 50 and output node44.

The active elements just described (i.e., transistor 56 and theinverting amplifier A) are formed in part in a semiconductor substrate58, which conventionally will have an upper epitaxial layer withisolated active areas formed therein. It will be appreciated that thepixel 40 is a small part of a larger fingerprint sensor that isfabricated as a specialized semiconductor integrated circuit chip. Thestarting material is typically a monocrystalline silicon wafer largeenough to include many identical fingerprint sensors that are eventuallyseparated by slicing the wafer into chips. The individual chip substrate58 has a composite dielectric body 60 disposed thereon and defining asensing surface 62 in the finished device. A dashed line 64schematically indicates a transition from the composite dielectric body60 to the substrate 58 below.

The composite dielectric body 60 includes multiple layers of dielectricor insulating material that are not shown separately. These dielectriclayers may include conventional oxide and nitride layers known to thoseskilled in the art of semiconductor device fabrication. Within thedielectric body 60 are several levels of conductive layers that aredeposited and patterned into separate conductors. Three such conductorsare shown in FIG. 3 as part of pixel 40 and are labeled: Surface Plate,Plate A and Plate B. The upper surface of the Surface Plate is flat andis preferably coplanar with surrounding flat surface portions of thedielectric body 60, together defining part of the sensing surface 62.Plate A and Plate B are embedded in the dielectric body 60 beneath theSurface Plate. Known photolithographic patterning techniques may be usedto form the plates and various suitable conductive materials may beused. For example, the Surface Plate may consist essentially of tungstenand Plates A and B may consist essentially of aluminum. Similarly-formedplates 66 and 68 of an adjacent pixel are partially shown at thebroken-off right edge of the dielectric layer 60.

The Surface Plate is shown contacting a portion of a user's finger 70,which includes fingerprint-bearing skin 72 having a typical fingerprintridge 74 and fingerprint valley 76. The portion of the skin 72 proximateto the Surface Plate defines a capacitor C_(f) whose capacitance varieswith the proximity of the skin to the Surface Plate. The capacitance ofCapacitor C_(f) is largest when a fingerprint ridge is in contact withthe Surface Plate, and is smallest when a fingerprint valley is directlyover the Surface Plate.

Plate A and the portion of the Surface Plate juxtaposed thereabovedefine a first coupling capacitor C_(a), and Plate B and the portion ofthe Surface Plate juxtaposed thereabove define a second couplingcapacitor C_(b). A capacitor C_(p) represents the parasitic capacitancebetween the Surface Plate and ground. Plate A is connected to theinverting amplifier's input node 50, and Plate B is connected to theinverting amplifier's output node 44.

It will be understood that the circuit elements designated C_(f), C_(a)and C_(b) in FIG. 3 merely show how the finger 70, the Surface Plate,Plate A and Plate B interact electrically; they are not separatestructural elements like the plates themselves. Likewise, capacitorC_(p) in FIG. 3 is parasitic and not a separate structural element.However, capacitor C_(input) is a separate structural element designedto have a particular capacitance value within manufacturing tolerances.Capacitor C_(input) may be formed in any suitable manner, and maycomprise, for example, metal plates at two different levels, or a metalplate disposed over a conductive polysilicon layer, or a conductivepolysilicon layer disposed over a heavily-doped surface region in thesubstrate 58.

The capacitance values of capacitors C_(a) and C_(b) should be as largeas practical to ensure good coupling of the variable capacitance ofcapacitor C_(f) across the inverting amplifier A. The space separatingthe Surface Plate from Plates A and B may be a conventional dielectricmaterial such as silicon nitride or silicon dioxide. Techniques forfabricating such layers are well known in the art. The dielectricmaterial separating the Surface Plate from Plate A and Plate B should beless than 1.0 micron thick, preferably having a thickness of about 0.2to 0.3 microns.

The transistor elements of the device, including transistor 56 and thetransistors that comprise the inverting amplifier A, are formed at theupper surface of the substrate, and typically include polysilicon gateelectrodes disposed within the composite dielectric body 60 just abovethe substrate 58. Because these conventional transistor elements arewell known, their structures are not specifically illustrated but areschematically depicted by the circuit diagram superimposed on thecross-sectional face of the substrate 58. It should also be understoodthat the parasitic capacitances C_(pi) and C_(po) arise from structuresthat exist in both the substrate 58 and in the dielectric body 60.

When a particular pixel is read, a pulse ΔV is generated by referencevoltage source V_(r). This pulse is applied to the pixel 40 through thepixel input node 42. The pulse ΔV propagates through capacitor C_(input)and appears at the input node 50 of the inverting amplifier A. Prior toreading the pixel 40, switch 56 is opened. The inverting amplifier Agenerates an output signal at its output node 44, which is communicatedto the output circuitry 48. The output circuitry digitizes the pixeloutput signal on node 44 for communication off the chip together withother pixel outputs in sequence for further processing. The analog valueof the output signal on output node 44 is determined by the gain of theinverting amplifier A, which is a function of the capacitance in itsfeedback loop. The series-connected capacitors C_(a) and C_(b) definethe feedback loop and their combined value is modulated by thecapacitance of the fingerprint capacitor C_(f).

An alternative circuit arrangement is shown in FIG. 4, the pixeldepicted therein being designated by reference numeral 80 to distinguishit from the pixel 40 of FIG. 3. However, like elements are designated bythe same reference characters. It will be noted that pixel 80 of FIG. 4has Plate A connected to the pixel input node 42, and Plate B connectedto the input node 50 of the inverting amplifier A. The input capacitorC_(input) of FIG. 3 is eliminated. In the circuit of pixel 80 in FIG. 4,a feedback capacitor C_(fb) is connected across the invertingamplifier's input node 50 and output node 44. The feedback capacitorC_(fb) is designed to have a particular capacitance value withinmanufacturing tolerances, which affects the gain of inverting amplifierA and causes it to function as a charge integrator. The feedbackcapacitor C_(fb) may be formed in any suitable manner, and may comprise,for example, metal plates at two different levels, or a metal platedisposed over a conductive polysilicon layer, or a conductivepolysilicon layer disposed over a heavily-doped surface region insubstrate 58.

In the alternative arrangements of FIGS. 3 and 4, the respective pixels40 and 80 each have sensing elements defined by four capacitors: C_(a),C_(b), C_(f) and C_(p). These four capacitors are shown generalized inthe FIG. 5 with connecting nodes: A, B, F, and ground. Nodes A and Bcorrespond to Plates A and B in FIGS. 3 and 4. FIG. 5 also shows thatcapacitors C_(a) and C_(b) share a common node 82, which corresponds tothe Surface Plate. Node F corresponds to the fingerprint-bearing skin72, which defines the variable fingerprint capacitor C_(f) with theSurface Plate. Parasitic capacitor C_(p) is connected between node 82and ground.

FIG. 6 further generalizes the arrangement of the Surface Plate andPlates A and B and the capacitors they define. By comparison with FIG.5, FIG. 6 depicts a generalized Sensing Element corresponding tocapacitors C_(a), C_(b), C_(f), and C_(p) showing only the connectingnodes: A, B, F, and ground.

FIG. 7 is a simplified circuit diagram corresponding to the pixel 40 ofFIG. 3, and FIG. 8 is a simplified circuit diagram corresponding to thepixel 80 of FIG. 4, both figures using the generalized Sensing Elementof FIG. 6, designated SE. FIGS. 7 and 8 show alternative circuitapplications for the Sensing Element embodiments of the presentinvention. In FIG. 7 the Sensing Element SE is connected in the feedbackloop of the inverting amplifier A, whereas in FIG. 8 the Sensing ElementSE is in the input path between the pixel's input node 42 and theinverting amplifier's input node 50. Various embodiments of theinventive Sensing Element of the present invention will now be describedin conjunction with FIGS. 9–12.

Referring to FIG. 9, a basic form of the Sensing Element useful ineither pixel 40 or 80 is shown in schematic cross section. Only theupper dielectric portion 60 of the pixel is shown. The substrate (notshown) will be understood to be below the dashed line 64. In thisembodiment, it is important for the dielectric material 84 separatingthe Surface Plate from Plates A and B therebelow to have a minimumpractical thickness. Preferably, the separating dielectric 84 should beabout 0.2 to 0.3 microns thick. A minimum practical thickness for theseparating dielectric 84 maximizes the capacitive coupling between theSurface Plate and Plates A and B therebelow.

Referring to FIG. 10, an alternative embodiment of the inventive SensingElement is shown in which an additional Plate C has been laterallyinterposed between Plates A and B. Plate C is connected to the SurfacePlate by a conductor 86, such as a metal via. This permits use of a muchthicker dielectric body portion 88 separating the Surface Plate from theplates therebelow. For example, dielectric body portion 88 can begreater than ten microns thick and preferably is from 15 to 20 micronsthick. Plates A, B and C are coplanar and have dielectric spacers 90 and92 separating the edges of Plate C from the respective opposed edges ofPlates A and B, as shown. Plates A, B and C can be etched from the sameconductive layer, such as an aluminum metalization, cutting 1.0 microngaps therein to define the separate plates. Thus, the capacitors C_(a)and C_(b) of FIG. 5 are formed between the facing edges of the embeddedPlates A, B and C. This effect is illustrated more clearly by FIG. 10A.

In FIG. 10A, a capacitor C_(a1) is formed between Plate A and Plate C,and a capacitor C_(a2) is formed between Plate A and the Surface Plate.If the thickness of the dielectric material separating the Surface Platefrom Plate A (preferably from 15 to 20 microns) is very much greaterthan the thickness of the dielectric material separating Plate A fromPlate C (preferably 1.0 micron), capacitor C_(a1) can be made largerthan capacitor C_(a2). Similarly, capacitor C_(b1) formed between PlateB and Plate C can be made larger than capacitor C_(a2). It will beappreciated that the layouts of the Surface Plate and embedded Plates A,B and C can be designed to insure that capacitors C_(a2) and C_(b2) arenegligible relative to capacitors C_(a1) and C_(b1). One such layout isdepicted in FIG. 10B.

FIG. 10B shows an interdigitated arrangement for Plates A, B and C ofFIG. 10A. Plates A and B have inwardly extending fingers 94 and Plate Chas outwardly extending fingers 96, as the plan view layout of FIG. 10Bshows. The multiple interdigitated fingers 94 and 96 form the facingedges that define the capacitors C_(a1) and C_(b1) of FIG. 10A. Such aninterdigitated arrangement greatly increases the capacitance ofcapacitors C_(a1) and C_(b1), since the shared area of the respectivecapacitor plates is dramatically increased.

The layout of FIG. 10B is just one of many possible alternatives thatcan increase the capacitance between adjacent plates within the limitsof a pixel's dimensions. For example, FIG. 8 of Tartagni et al. U.S.Pat. No. 6,114,862 shows a C-shaped plate and a complementary adjacentplate arrangement that can be converted into three adjacent plates byreplicating a mirror-image layout of the two adjacent plates. The resultis easily envisioned as a C-shaped plate on the left, a mirror-imagedC-shaped plate on the right, and a center plate therebetween, the centerplate having a common bar-shaped portion in the middle and extensionsthrough the C-shaped gaps to adjoining dumbbell-shaped ends within theC-shaped plates. The extensions and dumbbell-shaped ends are like twooppositely oriented fingers 96 of the interdigitated layout of FIG. 10Band serve the same capacitive enhancing function in a similar way.

Referring again to FIGS. 9 and 10, it will be appreciated that the platearrangement of FIG. 10 permits use of a very thick dielectric portion 88separating the Surface Plate from the lower embedded plates compared tothe relatively thin separating dielectric portion 84 required by thearrangement of FIG. 9. This is made possible by including Plate C in thearrangement of FIG. 10 and interconnecting it with the Surface Plateusing conductor 86. Note also that the Surface Plate need not bejuxtaposed directly over Plates A and B. The formation of capacitorsC_(a) and C_(b) using embedded Plate C connected to the Surface Plateallows one to decouple the layout geometry of the Surface Plate from theembedded plates therebelow. This is also true of the structures of FIGS.11 and 12. The thick dielectric portion 88 may be a hard glass materialthat can be deposited using conventional CVD techniques. Also, a thinsilicon nitride or oxynitride layer can be used atop a thicker glasslayer, which together can form the thick dielectric portion 88.

FIGS. 11 and 12 show additional embodiments of the inventive SensingElement using different arrangements of lower embedded plates. These twoarrangements use three levels of conductors. The embedded lower levelplates in FIGS. 11 and 12 can consist essentially of aluminum or cancomprise other suitable conductive materials.

In FIG. 11, Plate C is included beneath Plates A and B. Plate C isconnected to the Surface Plate by conductor 86. The dielectric materialportion 88 separating the Surface Plate form the underlying Plates A andB can be relatively thick, while a relatively thin dielectric materialportion or layer 98 separates Plate C from Plates A and B. Therefore,capacitors C_(a) and C_(b) of FIG. 5 are effectively formed betweenPlate C and Plates A and B, respectively.

FIG. 12 shows an arrangement similar to FIG. 11 with Plate C in thiscase located in the second level and Plates A and B located in the thirdlevel of conductive plates. Again, Plate C is connected to the SurfacePlate by conductor 86 so that the dielectric material 88 can be madevery thick while the dielectric material 98 is made relatively thin. InFIGS. 11 and 12, dielectric material 98 should be less than 1.0 micronthick and preferably is from 0.2 to 0.3 micron thick, whereas dielectricmaterial 88 should be greater than 10.0 microns thick and preferably isfrom 15 to 20 microns thick.

It will be appreciated that various additional sensor element structurescan be envisioned from the foregoing description. Features from FIGS.10–12 can be combined to further improve capacitive coupling, or tofacilitate layout-related or process-related design requirements. Forexample, Plate C in FIG. 11 or 12 can include an additional common platelying coplanar with Plates A and B and interposed therebetween in themanner of Plate C of FIG. 10.

Moreover, other materials than those described herein can be employed.For example, the Surface Plate can be formed from a titanium oxide filmrather than tungsten. Additionally, composite conductive layers can beused. For example, in the structure of FIG. 12, Plate C can be formed bya layer of titanium oxide atop a layer of aluminum, and Plates A and Bcan be aluminum or can be formed from a conventional conductivepolysilicon layer, such as silicided polysilicon.

Although preferred embodiments have been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. In a fingerprint sensor having a plurality of pixels for sensing afingerprint of a user's finger, each pixel having a sensing elementcoupled to an amplifier for generating an output corresponding to afingerprint characteristic appearing at the pixel location, the sensingelement comprising: a body of dielectric material; a surface plate ofconductive material retained by the dielectric body and having an uppersurface defining a sensing surface for contact with thefingerprint-bearing skin of a user's finger; first and second coplanarelectrodes embedded in the dielectric body at a first level below thesurface plate; and a third electrode embedded in the dielectric body ata second level below the surface plate, the third plate being connectedby a conductor to the surface plate; wherein a first sensor is formed bythe third electrode and the first electrode, a second sensor is formedby the third electrode and the second electrode, and a fingerprintsensor is formed between the surface plate and the skin of the user'sfinger thereover during a sensing operation.
 2. The sensing element ofclaim 1, wherein the first level is above the second level.
 3. Thesensing element of claim 2, wherein the thickness of the portion of thedielectric body separating the surface plate from the first and secondelectrodes is at least ten times greater than the thickness of theportion of the dielectric body separating the first and secondelectrodes from the third electrode therebelow.
 4. The sensing elementof claim 1 wherein the first level is below the second level.
 5. Thesensing element of claim 4, wherein the thickness of the portion of thedielectric body separating the surface plate from the third electrode isat least ten times greater than the thickness of the portion of thedielectric body separating the third electrode from the first and secondelectrodes therebelow.
 6. The sensing element of claim 1, wherein thedielectric body includes a thin layer separating the third electrodefrom the first and second electrodes, the thin dielectric layer having athickness of less than one micron.
 7. The sensing element of claim 6,wherein the thickness of the thin dielectric layer is from 0.2 to 0.3micron.
 8. The fingerprint element of claim 1, wherein said first,second and third electrodes comprise capacitor plates, and said firstand second sensors comprise first and second capacitors.